Recent studies have demonstrated that Vitamin D is a critical factor in the development of these cells that suppress allergic reactions. People with CF, due to their pancreatic insufficiency that causes them to have difficulty absorbing fat, also have lower levels of the fat soluble vitamins which include vitamin D. In the study done at UPMC, "Mechanisms of Immune Tolerance in ABPA", people with CF and ABPA had significantly lower vitamin D levels than people with CF who did not have ABPA.